Here are the Top 10 Airport Troubleshooting tips from Apple:
1) Make Sure That AirPort is On
If you can't make a connection, open Internet Connect in the Applications folder, click AirPort, and make sure that the dialog shows that AirPort is on. If it's off, click Turn AirPort On. Alternatively, if you've opted to display the AirPort menu in your menu bar, click the AirPort menu icon and be sure that AirPort: On is displayed in the menu. If not, choose Turn AirPort On.
2) Check Your Physical Location
Being free from wires doesn't mean that you can go anywhere and still get a connection. The maximum theoretical range of a wireless network is 150 feet, but range can vary based on your environment. If your connection seems spotty, make sure that there are no metal objects in the path between your computer and your base station or wireless access point. Try moving closer to your base station. Be aware that certain electrical items such as power lines, railroad tracks, and power stations, can cause communication interference—and try to avoid life situations that have you working behind concrete barriers or bullet-proof glass, both of which can impede your reception.
3) Check Your Access Privileges
If you enabled encryption on your network, make sure that you set up your base station or router properly, consult the documentation that came with your product, and that you're using the correct settings and password. If you're trying to connect to a wireless network that you didn't set up and are having difficulty, the network may require a password to access it, it will ask for one if required. If you're joining a closed network, a network with its name hidden, make sure that you've entered the network name and password correctly. If the network is secured by access control, make sure that your MAC address is registered with the network administrator—consult the network administrator for assistance.
4) Make Sure That the AirPort Port Is On
If you can't make a connection, make sure that your computer's AirPort port is active. From the Apple menu, choose System Preferences, then click Network. From the Show pop-up menu, choose Network Port Configurations (or Active Network Ports) and make sure that the AirPort checkbox is selected.
5) Restart Your Base Station or Wireless Router
Try restarting your base station or router by turning it off, waiting a few seconds, and then turning it back on. Or unplug the power cord, wait a few seconds, and then plug the cord back in. Then try connecting again.
6) Make Sure That Your AirPort Card Is Installed Correctly
If you installed an AirPort card into your computer but don't see AirPort in the Network pane of System Preferences or in Internet Connect, you may not have installed it properly, although this doesn't apply to computers that came with AirPort preinstalled. See the installation instructions that came with your card. To verify that your computer recognizes the card, choose About This Mac from the Apple menu and then click More Info to open System Profiler. You should see “AirPort Card” listed in the Contents list, either under Hardware or Network. If you don't, shut down your computer and reinstall the card. If that doesn't work, try installing another card.
7) Check for Other 2.4 GHz or 5.0 Ghz Electronics
If your wireless connection is intermittent, for example it works but may sometimes drop out, make sure that you're not using other devices that use the same 2.4 or 5.0 GHz frequency ranges as AirPort technology. 2.4 GHz cordless phones, baby monitors, microwave ovens, Bluetooth devices, and even some digital cameras can interrupt your connection, decrease performance, or decrease your AirPort signal range. Avoid using these types of devices when you need a continuous connection, such as when you're downloading large files. If you can, place your computer and yourself somewhere with less interference.
8) Check for Firmware Updates
Check the manufacturer's website for your base station or wireless router to see if there are any firmware updates available for your model. If so, download and install the software and try connecting again.
9) Verify That Your ISP is Working
If your Internet Service Provider's (ISP) servers are down, you won't be able to connect to the Internet. Make sure that this isn't the actual problem—open a web browser on any computer that's directly connected (by wire) to a modem or network and make sure that you can connect. If not, wait for your service to be restored.
10) Restart Your Computer
Sometimes a simple restart can clear up issues. From the Apple menu, choose Restart and restart your computer. Once it's finished starting up, try connecting to the network again.